Supplemental inflatable restraints (SIRs) are commonly used in motor vehicles to deploy air bags in the event of a crash for occupant protection. A frontal impact sensor, typically an accelerometer, develops a signal in the event of a crash, and a microprocessor evaluates the signal to decide whether to deploy the air bags. An arming circuit independently senses the crash event to verify that a crash is in progress and inhibits deployment in the absence of crash evidence. A firing circuit responds to a deployment command from the microprocessor to cause bag inflation. Typically the bags are in front of the occupants and the impact sensor and the arming circuit are sensitive to acceleration in the longitudinal direction of the vehicle. Now side air bags are employed to protect against side impact; side impact sensors in the vehicle doors send deploy messages to the microprocessor. Another arming circuit including a lateral accelerometer sensitive to acceleration in the lateral direction of the vehicle provides an arming signal to the microprocessor.
The microprocessor uses a robust algorithm to assess the frontal impact sensor output and decides whether to deploy, and also processes the frontal arming signal as well as the side deploy messages. To maintain this level of activity and meet the demands for rapid execution of all these functions, it is desirable to avoid any significant processing burden imposed by the lateral arming circuit.